This enzyme, found in green sulfur bacteria (Chlorobiaceae) and green flimentous bacteria (Chloroflexaceae), is a radical S-adenosyl-L-methionine (AdoMet) enzyme and contains a [4Fe-4S] cluster. It adds a methyl group at the C-121 position of bacteriochlorophylls of the c, d and e types. This methylation plays a role in fine-tuning the structural arrangement of the bacteriochlorophyll aggregates in chlorosomes and therefore directly influences the chlorosomes absorption properties.